Methods of fabrication for flip-chip image sensor packages

ABSTRACT

The present invention provides flip-chip packaging for optically interactive devices such as image sensors and methods of assembly. In a first embodiment of the invention, conductive traces are formed directly on the second surface of a transparent substrate and an image sensor chip is bonded to the conductive traces. Discrete conductive elements are attached to the conductive traces and extend below a back surface of the image sensor chip. In a second embodiment, a secondary substrate having conductive traces formed thereon is secured to the transparent substrate. In a third embodiment, a backing cap having a full array of attachment pads is attached to the transparent substrate of the first embodiment or the secondary substrate of the second embodiment. In a fourth embodiment, the secondary substrate is a flex circuit having a mounting portion secured to the second surface of the transparent substrate and a backing portion bent over adjacent to the back surface of the image sensor chip.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of application Ser. No. 10/375,864,filed Feb. 26, 2003, now U.S. Pat. No. 6,964,886, issued Nov. 15, 2005,which is a divisional of application Ser. No. 10/230,654, filed Aug. 29,2002, now U.S. Pat. No. 6,885,107, issued Apr. 26, 2005.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to packaging for optically interactiveelectronic devices. More particularly, the present invention relates toa package for a solid-state image sensor particularly suitable for highinput/output (I/O) count devices wherein an image sensor chip is mountedin flip-chip fashion to a transparent substrate or to a secondarysubstrate secured to the transparent substrate.

2. State of the Art

Optically interactive electronic devices, for example, charge coupleddevice (CCD) image sensors or complementary metal-oxide semiconductor(CMOS) image sensors, are typically packaged within a housing forsubsequent connection to higher-level packaging such as a larger circuitassembly in the form of a carrier substrate. The housing provideselectrical interconnection to the larger circuit assembly, providesprotection from the surrounding environment and allows light or otherforms of radiation to pass through to sensing circuitry located on theimage sensor device. In the prior art, package formation has often beenaccomplished by placing an image sensor device in the form of asemiconductor chip into the cavity of a plastic or ceramic housing, wirebonding electrical connection points on the semiconductor chip toconductive pads associated with the housing and sealing a window ortransparent cover over the cavity. The materials and structure involvedwith this packaging technique require a fabrication process that can betime consuming and requires several precision assembly steps. Wirebonding, for instance, involves special considerations during packageformation due to the fragile nature of bond wires and their associatedconnection points, and also may call for excessive package depth orthickness in order to accommodate the arched wire bond loops within thepackage cavity. Further, each assembly step increases the opportunityfor contamination or damage to the image sensor device itself, raisingdefect levels and slowing production time to avoid such damage andcontamination. Due to the extremely cost-competitive nature of today'ssemiconductor industry, even small improvements in product yield andproduction time are of value, especially when considered in terms of thehigh volume of components being manufactured.

In response to the above-noted shortcomings of the existing packagingtechniques, various structures using flip-chip mounting of an imagesensor chip have been developed in an attempt to simplify theconstruction of image sensor packages. U.S. Pat. No. 6,144,507 toHashimoto and U.S. Pat. No. 5,867,368 to Glenn, for example, eachdisclose an image sensor chip mounted directly to a printed circuitboard (PCB). An image sensor chip is mounted in flip-chip fashion overan aperture within the PCB, and a transparent cover is either attacheddirectly to the active surface of the chip or bonded to the side of thePCB opposite that to which the image sensor chip is attached and overthe aperture. Although these methods eliminate the difficultiesassociated with wire bonding and forming a housing for the image sensorchip, the illustrated PCBs are very large with respect to the size ofthe image sensor chip and the transparent cover. It is unclear from theaforementioned patents whether the PCBs comprise discrete image sensorpackages suitable for attachment to a larger circuit assembly, orthemselves comprise large circuit assemblies simply having an imagesensor chip mounted directly thereto, without the benefit of a packageor housing.

Another packaging approach has been to use the transparent cover itselfas a foundation for an image sensor package. U.S. Pat. No. 5,786,589 toSegawa et al. uses this approach by adhesively bonding a TAB sheet to aglass substrate and bonding an image sensor chip to the TAB tape with ananisotropically conductive film. This design requires a specializedsubstrate attachment technique due to the TAB-type connection leads.Moreover, the conductive film risks interference with sensing circuitryon the image sensor chip and requires the formation of dummy leads ordam structures to compensate for this problem. U.S. Pat. No. 6,342,406to Glenn et al. and U.S. Pat. No. 5,352,852 to Chun each bond an imagesensor chip to a transparent substrate by forming conductive tracesdirectly on the substrate. Glenn et al., however, uses a ball mountingarrangement on the first surface of the transparent substrate thatrequires the formation of vias through the substrate to connect interiorand exterior traces formed thereon. The arrangement also requires anaperture be formed within any substrate carrying the package to provideoptical access to the active surface of the image sensor chip, as theimage sensor active surface faces the attachment side of the package.Chun uses a lead arrangement that requires the package to be mountedwithin a substrate mounting depression or the addition of outleads whichmay be susceptible to damage during package handling and may not besuitable for high I/O sensor devices. Chun further requires the use of aframe of insulating tape to space the image sensor chip from theinterior of the transparent substrate.

As is evident from the foregoing review of the current state of the artin image sensor packaging, a need exists for an improved image sensorpackaging structure that is simple to fabricate, suitable for use withhigh I/O sensor devices, durable and easily mounted to a larger circuitassembly without the use of special substrate apertures or depressions.

SUMMARY OF THE INVENTION

In accordance with the present invention, image sensor packaging havingthe above-described and other beneficial characteristics and methods forfabrication thereof are provided. An image sensor chip is flip-chipmounted to conductive traces on a first surface of a transparentsubstrate. The active surface of the image sensor chip is protected fromcontamination after mounting by depositing a bead of sealant around theperiphery of the image sensor chip between the active surface of theimage sensor chip and the first surface of the substrate, thuseliminating any need for additional damming structures or spacing framesas used in the prior art. Discrete conductive elements such as solderballs or columns are attached to ends of the conductive traces whichform an array pattern, the discrete conductive elements extendingtransversely from the conductive traces on the first surface to asubstantially common plane at a level beyond a back surface of the imagesensor chip. The resulting structure comprises a board-over-chip (BOC)package arrangement that provides high I/O connectivity for the imagesensor chip and is easily mounted to a carrier substrate such as aprinted circuit board (PCB) without requiring the formation of specialapertures therein.

In a first exemplary embodiment of the present invention, conductivetraces are formed directly on the first surface of the transparentsubstrate and an image sensor chip is flip-chip mounted to firstattachment points of the conductive traces. The conductive traces extendbeyond the periphery of the image sensor chip, and discrete conductiveelements are joined to or formed on second attachment points of theconductive traces in an array pattern around the image sensor chip.

In a second exemplary embodiment of the present invention, a secondarysubstrate having the conductive traces formed thereon is secured to thefirst surface of the transparent substrate. An image sensor chip isflip-chip mounted to first attachment points of the conductive traces.The conductive traces extend beyond the periphery of the image sensorchip and discrete conductive elements are formed in an array pattern asin the first exemplary embodiment.

In a third exemplary embodiment of the present invention, a backing capis placed over the back surface of the image sensor chip. The side ofthe backing cap facing toward the image sensor chip carries contactsplaced in electrical communication with the conductive traces formed onthe transparent substrate or the secondary substrate which are, in turn,in electrical communication with the image sensor chip. Because thebacking cap covers the back surface of the image sensor chip, discreteconductive elements for connection to external circuitry may be formedin a full array pattern including the area underneath the image sensorchip.

In a fourth exemplary embodiment of the present invention, the secondarysubstrate secured to the transparent substrate is in the form of a flexcircuit having the conductive traces formed thereon. The flex circuitincludes a backing portion that is folded over the back surface of theimage sensor chip. The backing portion has discrete conductive elementsformed in a full array pattern as in the third exemplary embodiment.

In a further variation to any of the above-described packageembodiments, an array of image sensor packages may be simultaneouslyformed on one large transparent substrate, which is then cut orotherwise divided or singulated to form multiple individual image sensorpackages.

Other and further features and advantages of the present invention willbe apparent from the following descriptions of the various exemplaryembodiments read in conjunction with the accompanying drawings. It willbe understood by one of ordinary skill in the art that the following areprovided for illustrative and exemplary purposes only, and that numerouscombinations of the elements of the various embodiments of the presentinvention are contemplated as within the scope of the present invention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

In the drawings, which illustrate what is currently considered to be thebest mode for carrying out the invention:

FIG. 1 is a perspective view showing an image sensor chip containedwithin image sensor packaging according to the present invention;

FIG. 2 is a side view showing a transparent substrate according to thepresent invention;

FIG. 3 is a sectional side view showing an image sensor packageaccording to a first embodiment of the present invention;

FIG. 4 is an underside view showing a second surface of a transparentsubstrate for the image sensor package depicted in FIG. 3;

FIG. 5 is a side view showing the image sensor package depicted in FIG.3 mounted to a carrier substrate;

FIG. 6 is a flow chart showing a method of fabrication according to thefirst embodiment of the present invention;

FIG. 7 is a sectional side view showing an image sensor packageaccording to a second embodiment of the present invention;

FIG. 8 is a plan, underside view showing a secondary substrate for theimage sensor package depicted in FIG. 7;

FIG. 9 is a flow chart showing a method of fabrication according to thesecond embodiment of the present invention;

FIG. 10 is a sectional side view showing a backing cap for an imagesensor package according to a third embodiment of the present invention;

FIG. 11 is a top plan view of the backing cap depicted in FIG. 10;

FIG. 12 is a sectional side view showing an image sensor packageaccording to the third embodiment of the present invention;

FIG. 13 is a flow chart showing a method of fabrication according to thethird embodiment of the present invention;

FIGS. 14A and 14B are perspective views showing a flex circuit accordingto a fourth embodiment of the present invention;

FIGS. 15A and 15B are sectional side views showing an image sensorpackage according to the fourth embodiment of the present invention;

FIG. 16 is a flow chart showing a method of fabrication according to thefourth embodiment of the present invention; and

FIG. 17 is a flow chart showing a method of fabrication according to afurther variation of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring in general to the accompanying drawings, various aspects ofthe present invention are illustrated to show the structure and methodsfor assembly of an image sensor package formed on a transparentsubstrate. Common elements and features of the illustrated embodimentsare designated by the same or similar reference numerals. It should beunderstood that the figures presented are not meant to be illustrativeof actual views of any particular portion of the actual devicestructure, but are merely idealized schematic representations which areemployed to more clearly and fully depict the invention. It shouldfurther be understood that, while depicted in terms of an image sensor,the package embodiments and methods presented herein would work well forother types of optically interactive electronic devices. The term“optically interactive” as used herein encompasses devices sensitive tovarious wavelengths of light or other forms of radiation, such as, butnot limited to, CCD and CMOS image sensors, EPROMs, and photodiodes, aswell as light-emitting devices such as semiconductor lasers andlight-emitting diodes.

FIG. 1 shows a perspective view of an exemplary image sensor chip 2suitable for use with the various embodiments of the present invention.Image sensor chip 2 includes an active surface 4 and a back surface 6.Active surface 4 and back surface 6 are bounded by sides 8, also termedthe “periphery” of image sensor chip 2. Active surface 4 includessensing circuitry 10 in a central portion and bond pads 12 formedoutwardly of sensing circuitry 10 and around the perimeter of activesurface 4. As used herein, the term “bond pads” includes not only bondpads directly connected to underlying sensing circuitry 10 but also bondpad locations removed from original locations using a redistributionlayer extending over active surface 4 as known in the art. Bond pads 12are depicted in a single row along each of the four sides 8 of imagesensor chip 2, but other arrangements are possible, for example and notby way of limitation, having bond pads 12 formed along fewer sides or inmultiple rows along one or more sides. If multiple rows are used, thebond pads 12 of one row may be staggered or offset from those of anadjacent row. Conductive bumps 14 (only some shown by way of example)are formed on bond pads 12 to enable flip-chip attachment of imagesensor chip 2 and will be described in further detail below. For certaintypes of image sensors, micro-lenses 16 may be formed over sensingcircuitry 10 to aid in the reception of light.

FIG. 2 shows a side view of an exemplary transparent substrate 18serving as the primary package structure for the various embodiments ofthe present invention. Transparent substrate 18 includes a first surface20 and a second surface 22. Transparent substrate 18 may be formed ofglass, plastic or any other substrate material that is suitablytransmissive of light or other forms of radiation. Accordingly, the term“transparent” is not used in its literal sense, but is descriptive of asubstrate material exhibiting sufficient transmissivity of light atselected wavelengths sufficient for operation of image sensor chip 2. Aplate of borosilicate glass is one specific, and currently preferred,example of a suitable material for transparent substrate 18. Transparentsubstrate 18 may also provide an auxiliary optical function, forinstance, by forming first surface 20 into a lens, grating or othersuitable configuration to provide light focusing, refraction ordispersion capabilities or by using a material for or coating oftransparent substrate 18 that prevents selected wavelengths of lightfrom passing therethrough to sensing circuitry 10.

Turning to FIGS. 3 through 6, a first embodiment according to thepresent invention is illustrated. FIG. 3 shows a sectional side view ofan image sensor package 24 wherein conductive traces 26 (thicknessenlarged for clarity) are formed directly on second surface 22 oftransparent substrate 18. As seen in FIG. 4, which is an underside planview of second surface 22 as oriented in FIG. 3, conductive traces 26have first attachment points 28 formed at locations around the center oftransparent substrate 18 and corresponding to the spacing of bond pads12 (FIG. 1) on image sensor chip 2. Spreading out from the center oftransparent substrate 18, conductive traces 26 have second attachmentpoints 30 formed in an array pattern. For purposes of illustration,second attachment points 30 have been depicted in a single row runningaround the center and proximate the periphery of transparent substrate18. It should be understood, however, that the number of rows employedmay be based on factors such as the number and arrangement of bonds pads12 on image sensor chip 2, and may comprise several rows of secondattachment points 30. Conductive traces 26 may be formed on secondsurface 22 of transparent substrate 18 using a variety of techniques.For instance, a conductive or conductor-filled liquid epoxy or resincould be printed, screen-printed or otherwise dispensed onto secondsurface 22 in the desired pattern and then cured to form conductivetraces 26. Alternatively, a layer of conductive material such as metalor doped silicon may be formed over the entirety of second surface 22,and then selectively etched to form conductive traces 26. As yet anotherapproach, adhesively coated preformed traces carried by a removablebacking layer may be applied to transparent substrate 18 and then thebacking layer removed.

Returning to FIG. 3, an image sensor chip 2 is flip-chip mounted tofirst attachment points 28 by bonding with conductive bumps 14 such thatsensing circuitry 10 on active surface 4 is exposed through the centralportion of transparent substrate 18. The term “bumps” includes, withoutlimitation, conductive structures configured as balls, bumps, columns,pillars and studs. In the various embodiments of the present invention,conductive bumps 14 are formed of materials known in the art for use inflip-chip bonding. Suitable materials may include, but are not limitedto, conductive or conductor-filled epoxy, eutectic tin/lead solder andgold. Conductive bumps 14 may be formed of one material, not necessarilyconductive, and coated with another. Alternatively, a strip or segmentof an anisotropically conductive adhesive film may be employed in lieuof a plurality of conductive bumps 14. In the case where sensingcircuitry 10 is covered by micro-lenses 16, conductive bumps 14 areformed to a sufficient thickness so as to leave a vertical gap 32between micro-lenses 16 and second surface 22 to enable micro-lenses 16to properly receive light through transparent substrate 18.

FIG. 3 further shows discrete conductive elements 34 attached to secondattachment points 30 and extending downwardly from transparent substrate18 to a plane at a level below back surface 6 of image sensor chip 2. Byextending beyond the back surface 6 of image sensor chip 2, discreteconductive elements 34 enable image sensor package 24 to be mounted to acarrier substrate 38 (FIG. 5) without the need for a depression orcavity within the surface of carrier substrate 38 to accommodate imagesensor chip 2. Moreover, since discrete conductive elements 34 interfacewith carrier substrate 38 below or “behind” image sensor package 24,active surface 4 of image sensor chip 2 faces outwardly. Therefore, anaperture is not required within carrier substrate 38 to expose activesurface 4. In a presently preferred embodiment, discrete conductiveelements 34 are structures that extend downwardly in a directionperpendicular to the plane of second surface 22, such as solder balls orcolumns. These structures are less susceptible to damage during handlingthan the lead arrangements used in prior art packaging. They may also beformed in multiple rows, making them more suitable for packaging of highI/O devices. The solder balls or columns may be formed entirely of aeutectic tin/lead solder, or may have a solid metal core surrounded bysolder. Forming discrete conductive elements of other materials such asconductive or conductor-filled epoxy or anisotropically conductivematerials is also within the scope of the present invention, as long asthey can be sized and configured to meet the desired height and pitchrequirements.

A bead of sealant 36 is deposited around the sides 8 or periphery ofimage sensor chip 2 and contacts second surface 22 of transparentsubstrate 18. Sealant 36 protects active surface 4 of image sensor chip2 from outside environmental damage and further mechanically secures theintegrity of the flip-chip bonds extending between bond pads 12 of imagesensor chip 2 and conductive traces 26 of transparent substrate 18.Because sealant 36 is placed around the periphery of active surface 4after attachment of image sensor chip 2, there is no need for additionaldamming structures or spacing frames which complicate the fabricationprocess. Nonexhaustive examples of suitable sealant 36 material includea viscous liquid or gelled epoxy or silicone deposited around imagesensor chip 2 and cured in place.

FIG. 5 shows a side view of image sensor package 24 mounted to carriersubstrate 38, wherein discrete conductive elements 34 are formed assolder balls which have been reflowed to bond to conductive terminalpads 40 on the surface of carrier substrate 38.

FIG. 6 shows a flow chart of an exemplary method of fabrication for thefirst embodiment of the present invention. First, in action 100,conductive traces 26 are formed on second surface 22 of transparentsubstrate 18. In action 102, image sensor chip 2 is flip-chip mounted bybonding conductive bumps 14 to first attachment points 28. Next, inaction 104, a bead of sealant is deposited around image sensor chip 2,contacting sides 8 and transparent substrate 18. Finally, in action 106,discrete conductive elements 34 are formed on or attached to secondattachment points 30, and image sensor package 24 is completed.

A second embodiment according to the present invention is illustrated inFIGS. 7 through 9. FIG. 7 shows a sectional side view of an image sensorpackage 42. A secondary substrate 44 having aperture 46 and conductivetraces 26 (enlarged for clarity) formed thereon is secured to secondsurface 22 of the transparent substrate 18, and image sensor chip 2 isflip-chip mounted to secondary substrate 44. Secondary substrate 44 maybe, for example, a PCB formed of an FR-4 or FR-5 laminate, a BT epoxyresin, a ceramic, or silicon with conductive traces 26 formed thereon byconventional PCB fabrication techniques, such as the aforementionedprinting, etching, etc. The secondary substrate 44 is then secured totransparent substrate 18 such as with an adhesive material applied tothe PCB and/or transparent substrate 18. Alternatively, secondarysubstrate 44 may be formed of a polymer film such as a polyimide havingconductive traces formed on one side and an adhesive coating on theopposite side for securing to transparent substrate 18. Apressure-sensitive, heat or light-curable adhesive may be employed.Nonadhesive bonding, such as heat bonding, of secondary substrate 44 totransparent substrate 18 may be employed with suitable materials.

As seen in FIG. 8, which is a plan underside view of secondary substrate44, conductive traces 26 have first attachment points 28 and secondattachment points 30, and spread out from aperture 46 in a patternsimilar to that of the first embodiment. While depicted as being on anoutside surface of secondary substrate 44 facing image sensor chip 2,conductive traces 26 may reside within secondary substrate 44 or belocated on the opposite side thereof and include interlevel connections(vias) respectively extending to the first and second attachment points28 and 30. Conductive bumps 14 on image sensor chip 2 are bonded tofirst attachment points 28 such that sensing circuitry 10 is exposedthrough aperture 46. Discrete conductive elements 34 having the samestructure as in the first embodiment are attached to second attachmentpoints 30. A bead of sealant 36 is also deposited around the sides 8 ofimage sensor chip 2, but sealant 36 contacts secondary substrate 44rather than transparent substrate 18 as in the first embodiment.

FIG. 9 shows a flow chart of an exemplary method of fabrication for thesecond embodiment of the present invention. First, in action 200,secondary substrate 44 having aperture 46 and conductive traces 26 isadhesively secured to transparent substrate 18. In action 202, imagesensor chip 2 is flip-chip mounted by bonding conductive bumps 14 tofirst attachment points 28. Next, in action 204, a bead of sealant 36 isdeposited around image sensor chip 2, contacting sides 8 and secondarysubstrate 44. Finally, in action 206, discrete conductive elements 34are formed on or attached to second attachment points 30, and imagesensor package 42 is completed.

For image sensor devices with higher I/O requirements, a full arraypattern of discrete conductive elements may be desirable for attachingan image sensor package to a carrier substrate. Turning to FIGS. 10through 13, a third embodiment of the present invention providing a fullarray pattern is illustrated. FIG. 10 shows a sectional side view of abacking cap 48 that is constructed to extend over back surface 6 andsides 8 of image sensor chip 2. Backing cap 48 includes base 50 andsidewalls 52 around and extending transversely from the perimeter ofbase 50. Base 50 and sidewalls 52 may be formed, for example, ofconventional PCB materials such as glass-reinforced fiber, polymeric orceramic materials. Conductive lines 54 extend through backing cap 48from attachment points 56 exposed on sidewalls 52 to attachment pads 58exposed on base 50. Because attachment pads 58 are formed on base 50,they may be arranged in a fully populated array across the entire areaof “footprint” of the image sensor package, providing more I/Ointerfaces for a sensor device contained therein.

Backing cap 48 may be used in combination with elements of both imagesensor package 24 of the first embodiment and image sensor package 42 ofthe second embodiment. Rather than attaching discrete conductiveelements 34 to second attachment points 30, backing cap attachmentpoints 56 may be bonded to second attachment points 30 of conductivetraces 26 for electrical communication with attachment pads 58. Secondattachment points 30 and backing cap attachment points 56 may be bondedin a variety of ways, such as by application of conductive orconductor-filled epoxy or by forming a solder joint between the two. Aframe comprising a film of anisotropically conductive adhesive materialmay also be applied between sidewalls 52 and transparent substrate 18 orsecondary substrate 44, depending on the image sensor package embodimentelements used with backing cap 48. This approach electrically connectsand bonds second attachment points 30 and backing cap attachment points56, and further acts to seal backing cap 48 into place. With the thirdembodiment, second attachment points 30 may be formed much smaller thanin the first two embodiments, as they are not required to support theformation of discrete conductive elements 34. Accordingly, an increasednumber of second attachment points 30 may be patterned around theoutside of image sensor chip 2 for bonding with a corresponding numberof backing cap attachment points 56. This makes it possible toelectrically communicate with a fully populated array of attachment pads58 for higher I/O connectivity. FIG. 11 shows a top plan view of backingcap 48 with an exemplary single row pattern of backing cap attachmentpoints 56. It should also be noted that conductive lines 54 may beformed to extend from backing cap attachment points 56 and around theexterior of sidewalls 52 and over the back surface of base 50 toattachment pads 58. Similarly, for simplicity, the portions ofconductive lines 54 extending through sidewalls 52 may comprise viasleading to a redistribution layer of conductive traces formed over theback surface of base 50.

FIG. 12 shows a sectional side view of an image sensor package 60according to the third embodiment of the present invention using atransparent substrate 18 with conductive traces 26 formed directly onsecond surface 22, as in the first embodiment. After electricalconnection is made between backing cap attachment points 56 and secondattachment points 30 of conductive traces 26, backing cap 48 is sealedto transparent substrate 18 with a layer or bead of dielectric adhesive62. Alternatively, and as previously noted, an anisotropicallyconductive adhesive may be used to both electrically bond secondattachment points 30 and backing cap attachment points 56 and sealbetween backing cap 48 and transparent substrate 18. Discrete conductiveelements 34 are attached to or formed on attachment pads 58. Discreteconductive elements 34 may be formed as structures and of materialssimilar to those described in conjunction with the first and secondembodiments of the present invention, but will not require as large aheight as they are already located at a point below image sensor chip 2.Alternatively, since attachment pads 58 reside on a plane on the secondsurface of image sensor package 60, discrete conductive elements 34 maybe omitted and attachment pads 58 directly attached to conductiveterminal pads 40 on carrier substrate 38 in a land-grid array-typearrangement.

FIG. 13 shows a flow chart of an exemplary method of fabrication for thethird embodiment of the present invention. First, in action 300,conductive traces 26 are formed on second surface 22 of transparentsubstrate 18 when using the packaging structure of the first embodiment.Alternatively, in action 300′, secondary substrate 44 having aperture 46and conductive traces 26 is adhesively secured to transparent substrate18 when using the packaging structure of the second embodiment. Inaction 302, image sensor chip 2 is flip-chip mounted by bondingconductive bumps 14 to first attachment points 28. In action 304, a beadof sealant is deposited around image sensor chip 2, contacting sides 8and transparent substrate 18 or secondary substrate 44 depending on thepackage structure used. Because image sensor chip 2 will be covered andsealed by backing cap 48, the sealant of action 304 may optionally be,and preferably is, omitted. Next, in action 306, backing cap 48 issecured in place and second attachment points 30 and backing capattachment points 56 are bonded for electrical communication. Finally,in action 308, if a land-grid array-type package is not desired,discrete conductive elements 34 are formed on or attached to the fullarray of attachment pads 58.

A fourth embodiment of the present invention that is also capable ofproviding a full array of I/O interfaces is illustrated in FIGS. 14Athrough 16. The fourth embodiment of the present invention is avariation of the second embodiment, wherein secondary substrate 44 is inthe form of a flex circuit 64 having conductive traces 26 formedthereon. Flex circuit 64 is constructed of conventional flexible-circuitmaterials such as a flexible polyimide film having conductive traces 26formed on or within the film. As used herein, the term “flexible” meansany substrate material that may be substantially bent over itself orfolded without causing substantial damage to the flex circuit andspecifically the conductive traces 26 thereof. FIGS. 14A and 14B showexemplary second and first surface perspective views of flex circuit 64.Flex circuit 64 includes a mounting portion 66 and a backing portion 68.Mounting portion 66 includes aperture 46 as in the second embodiment.First attachment points 28 of conductive traces 26 are patterned aroundaperture 46 on a first side 70 of flex circuit 64. However, rather thanforming second attachment points 30 in a pattern around aperture 46,conductive traces 26 (only some shown for clarity) extend onto backingportion 68 through vias (see vias 27 in broken lines in FIGS. 15A and15B) and second attachment points 30 are formed in a fully populatedarray pattern on a second side 72 of flex circuit 64. It should beunderstood that conductive traces 26, while being depicted as straightlines in FIG. 14A for the purposes of illustration, would be routed suchthat each conductive trace 26 would terminate at a separate secondattachment point 30 on second side 72. Vias 27 may be etched into flexcircuit 64, filled by electroless or electroplating, or filled with aconductive or conductor-filled polymer, after which second attachmentpoints 30 are formed thereover as by, for example, formation orapplication of a conductive layer, patterning and etching.

FIG. 15A shows a sectional side view of an image sensor package 74according to the fourth embodiment of the present invention. Mountingportion 66 is adhesively secured to transparent substrate 18 in the samemanner as secondary substrate 44 in the second embodiment. Backingportion 68 extends laterally outwardly from an edge of transparentsubstrate 18. Conductive bumps 14 on image sensor chip 2 are bonded tofirst attachment points 28 such that sensing circuitry 10 is exposedthrough aperture 46 (FIGS. 14A and 14B). A bead of sealant 36 is alsodeposited around the sides 8 of image sensor chip 2 and contacts firstside 70 of flex circuit 64.

As seen in FIG. 15B, backing portion 68 is subsequently bent over orfolded such that the full array of second attachment points 30 faces ina downward direction below image sensor chip 2. In a presently preferredembodiment, a first side 78 of a rigid substrate 76 is adhesivelyattached to backing portion 68 on first side 70 of flex circuit 64 forsupport when image sensor package 74 is later placed on carriersubstrate 38. Rigid substrate 76 may be prefabricated with flex circuit64 before connection of image sensor chip 2 thereto, or may be attachedat a later point during formation of image sensor package 74. Rigidsubstrate 76 may be formed of any suitably rigid material. It may beformed, for example, of a metal (suitably electrically insulated fromconductive traces 26) which would also aid in heat dissipation whenattached to image sensor chip 2. Furthermore, rigid substrate 76 mayitself be formed with conductive traces with ends extending from secondattachment points 30 to locations over vias arranged to correspond withvias 27 and disposed on the outside of backing portion 68 with its viasand aligned with vias 27 on flex circuit 64. A second side 80 of rigidsubstrate 76 is further adhesively attached to back surface 6 of imagesensor chip 2 to hold flex circuit 64 in the folded position.Conventional adhesives may be used for attaching rigid substrate 76 toflex circuit 64 and image sensor chip 2. Alternatively or additionally,rigid substrate 76 may be held in place with stand-off structures 82attached between second side 80 thereof and first side 70 of flexcircuit 64 on mounting portion 66. Stand-off structures 82 may beintegrally formed with rigid substrate 76 (as by molding) and attachedto mounting portion 66, or may be separate structures attached by anyknown methods, for instance, by adhesive bonding or press-fitting intoapertures therein.

As with backing cap 48 in the third embodiment, discrete conductiveelements 34 may be attached to or formed on second attachment points 30,or may be omitted to form a land-grid array-type package.

FIG. 16 shows a flow chart of an exemplary method of assembly for thefourth embodiment of the present invention. First, in action 400,mounting portion 66 having aperture 46 and first attachment points 28 isadhesively secured to transparent substrate 18. In action 402, imagesensor chip 2 is flip-chip mounted by bonding conductive bumps 14 tofirst attachment points 28. In action 404, a bead of sealant isdeposited around image sensor chip 2, contacting sides 8 and first side70 of flex circuit 64. In action 406, backing portion 68 havingoptionally preattached rigid substrate 76 and a fully populated array ofsecond attachment points 30 is bent over and secured in place with anadhesive material previously applied to second side 80 of rigidsubstrate 76 and/or to back surface 6 of image sensor chip 2.Alternatively or additionally, backing portion 68 is secured by the useof stand-off structures 82. Finally in action 408, if a land-gridarray-type package is not desired, discrete conductive elements 34 areformed on or attached to the full array of second attachment points 30on second side 72 of flex circuit 64.

In a further variation to any of the above-described packageembodiments, depicted in the flow chart of FIG. 17, an array of imagesensor packages may be simultaneously formed on one large transparentsubstrate 18 (actions 500, 502), which is then cut or otherwise dividedto form multiple individual image sensor packages (action 504). Themethod of assembly for each of the embodiments may proceed in the samefashion as previously described, with singulation occurring, with theexception of the fourth embodiment, after attachment of discreteconductive elements 34 or backing cap 48. Of course, singulation may becarried out at an earlier point in the package fabrication process if sodesired. Further, it is contemplated that backing caps 48 might also beconstructed as a unitary array of backing caps 48 (molded, for example,with runners extending therebetween) which may then be divided alongwith the large transparent substrate 18 during singulation. In addition,if a secondary substrate is employed in accordance with the secondembodiment, it may be of like size and shape to that of the transparentsubstrate, applied thereto and singulated therewith.

All of the disclosed embodiments of the present invention providedurable image sensor packaging that is simple to fabricate, suitable forhigh I/O sensor devices and easily mounted to a larger circuit assemblywithout the use of special carrier substrate apertures or depressions.Although the present invention has been depicted and described withrespect to the illustrated embodiments, various additions, deletions andmodifications are contemplated within its scope or essentialcharacteristics. For instance, sealant 36 may be deposited to cover theentire sides 8 and back surface 6 of image sensor chip 2 for greaterprotection, or an additional deposition of another encapsulant materialcould be used for this purpose. Also, actions of the exemplary assemblymethods could be carried out in a different order, such as by attachingdiscrete conductive elements 34 to backing cap 48 or flex circuit 64 ina prefabrication process. Furthermore, while described in the context ofan image sensor package, the present invention has utility for thepackaging of numerous types of optically interactive electronic devices.The scope of the present invention is, therefore, indicated by theappended claims rather than the foregoing description. All changes whichcome within the meaning and range of equivalency of the claims are to beembraced within their scope.

1. A method for fabricating an electronic device package, comprising:forming a plurality of conductive traces on a surface of a transparentsubstrate, each conductive trace of the plurality of conductive traceshaving a first attachment point and a second attachment point; providingan optically interactive electronic device having at least one bond padon an active surface thereof; bonding the at least one bond pad of theoptically interactive electronic device to the first attachment point ofa conductive trace of the plurality of conductive traces; providing abacking cap having a base and sidewalls extending transversely from aperimeter of the base and at least one conductive pad on a first surfaceof the base thereof; and attaching the backing cap to the transparentsubstrate to cover a back surface of the optically interactiveelectronic device and provide electrical communication between thesecond attachment point of the conductive trace and the at least oneconductive pad on the first surface of the backing cap.
 2. The methodaccording to claim 1, wherein forming a plurality of conductive traceson a surface of a transparent substrate comprises forming the secondattachment points of the plurality of conductive traces in at least onerow extending adjacent a perimeter portion of the surface of thetransparent substrate.
 3. The method according to claim 2, furthercomprising: forming the second attachment points of the plurality ofconductive traces in multiple rows extending adjacent the perimeterportion of the surface of the transparent substrate.
 4. The methodaccording to claim 1, wherein forming a plurality of conductive traceson a surface of a transparent substrate comprises: disposing aconductive or conductor-filled material onto the surface of thetransparent substrate in a pattern defining the plurality of conductivetraces.
 5. The method according to claim 1, wherein forming a pluralityof conductive traces on a surface of a transparent substrate comprises:disposing a layer of conductive material on the surface of thetransparent substrate; and selectively etching the conductive materialto define the plurality of conductive traces.
 6. The method according toclaim 1, further comprising: depositing a bead of sealant material onthe surface of the transparent substrate along at least one side of theoptically interactive electronic device and in contact therewith.
 7. Themethod according to claim 6, wherein the sealant material is selected tocomprise one of epoxy and silicone.
 8. The method according to claim 1,wherein the optically interactive electronic device is selected tocomprise an image sensor.
 9. The method according to claim 1, furthercomprising: mounting a plurality of optically interactive electronicdevices on a single transparent substrate; and dividing the singletransparent substrate to provide individual electronic device packages.10. The method according to claim 1, wherein attaching the backing capto the transparent substrate comprises forming a bond between the atleast one backing cap conductive pad and the second attachment point ofthe conductive trace, the bond being selected to comprise one of aconductive or conductor-filled epoxy, a solder joint and a layer ofanisotropically conductive adhesive material.
 11. The method accordingto claim 1, further comprising: forming an array of attachment pads on asecond surface of the backing cap.
 12. The method according to claim 11,further comprising: providing a discrete conductive element on at leastone attachment pad of the array of attachment pads on the second surfaceof the backing cap.
 13. The method according to claim 12, wherein thediscrete conductive element is selected to comprise one of a solderball, a solder column, a conductive epoxy, and a conductor-filled epoxy.